Stray field rejection in magnetic sensors

ABSTRACT

A field-sensor device comprises a reference field sensor biased with a reference current. The reference field sensor provides a reference sensor signal in response to a field, and a calibrated field sensor biased with an individually adjustable current and providing a calibrated sensor signal in response to the field. A control circuit controls the adjustable current bias of the calibrated field sensor at a calibrated current different from the reference current so that the calibrated field sensor provides a calibrated sensor signal substantially equal to the reference sensor signal in response to a common field. The field sensor device is arranged to be exposed, when in a calibration mode, to a uniform calibration field and, when in operational mode, to an operational field being a field gradient.

FIELD OF THE INVENTION

The present invention is generally related to the domain of field sensordevices having improved stray field rejection and reduced variability.

BACKGROUND OF THE INVENTION

Sensors are widely used in electronic devices to measure attributes ofthe environment and report a measured sensor value. In particular,magnetic sensors are used to measure magnetic fields, for example intransportation systems such as automobiles. Magnetic sensors canincorporate Hall-effect sensors that generate an output voltageproportional to an applied magnetic field or magneto-resistive materialswhose electrical resistance changes in response to an external magneticfield. In many applications, it is desirable that sensors are small andare integrated with electronic processing circuitry so as to reduce theoverall sensor size and provide improved measurements and integrationinto external electronic systems. For example, US2016/299200 describes aHall-effect magnetic sensor for measuring magnetic fields incorporatingan integrated circuit formed in a semiconductor material on a substrate,together with insulation and adhesive layers.

Magnetic sensors are often used to detect the position of a movingmechanical structure, for example a rotating element incorporating amagnet (a magnetic position sensor). U.S. Pat. No. 9,523,589 describes arotation angle measurement apparatus having four Hall element pairs fordetecting magnetic field components in four different directions andused to calculate the position of a rotating magnet.

In any manufacturing process, materials and structures vary so that themanufactured devices vary somewhat. In particular, manufactured sensorscan provide slightly varying output when exposed to an environmentalattribute of the same magnitude. For example, magnetic Hall sensorsformed on a semiconductor wafer can produce signals that vary by severalpercent due to variations in the quality of epitaxial materials in thewafer and variations in the photolithographic process methods or due tothe local variation of mechanical stress, for example arising fromtemperature changes of packaged devices and a mismatch of the thermalcoefficients of expansion of the electronic circuit and its surroundingpackaging material in the packaged device.

Various approaches to improving the consistency of sensors and theirmeasurements are known. In one simple example, multiple, redundantsensors can be employed and their sensed values averaged to reduceoverall variability. Nonetheless, significant measurement errors arestill possible. In some sensor applications, the sensors can becalibrated or re-calibrated to ensure that their measurements conform toan external standard. However, in other applications such calibration isnot possible, difficult or too expensive.

Moreover, field sensors are subject to measurement error due to strayfields that are unrelated to the field whose measurement is desired. Forexample, a compass is intended to measure the earth's magnetic field butcan be affected by local magnetic field sources, such as motors or otherelectrical machinery that produce stray magnetic fields. Conversely, amagnetic sensor designed to measure the temporal variation of themagnetic field produced by a rotating magnet can be affected by theearth's magnetic field, fields from other unrelated electrical machineryor electromagnetic interference. Hence, stray field rejection is animportant attribute of practical field sensor designs, such as positionsensors.

One approach to reducing stray field rejection is to increase the usefulsignal from the sensor, for example by employing a magnet with astronger magnetization or by bringing the magnet closer to the sensor,so that the relative contribution of the undesired external field to thesensed signal is reduced. However, this can require larger, heavier orbulkier magnets and/or more accurate mechanical assembly with morestringent tolerances, thereby increasing size and/or cost. For example,one commercially available design requires magnets capable of producing70 mT magnetic fields. In contrast, many sensor applications requireincreased sensitivity, accuracy, precision and reduced costs.

There is a need, therefore, for improved field sensor devices thatprovide increased stray field rejection and improved accuracy.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide for afield sensor device wherein one or more of the above-mentioned drawbacksand limitations are avoided or overcome. It is also an object to providea method of calibrating such device. It is a further object to propose away of operating the field sensor device.

The above objective is accomplished by the solution according to thepresent invention.

Embodiments of the present invention provide a field-sensor devicecomprising a reference field sensor biased with a reference current, thereference field sensor providing a reference sensor signal in responseto a field, and a calibrated field sensor biased with an adjustablecurrent, the calibrated field sensor providing a calibrated sensorsignal in response to the field. When the adjustable current of thecalibrated field sensor is equal to the reference current, the referencesensor signal is not equal to the calibrated sensor signal (for exampledue to difference in materials, manufacturing or assembly). When thecalibrated field sensor is biased at a calibrated current different fromthe reference current, the calibrated field sensor provides a calibratedsensor signal substantially equal to the reference sensor signal. Thefield sensor device is arranged to be exposed, when in a calibrationmode, to a uniform calibration field and, when in operational mode, toan operational field being a field gradient.

In various configurations of the present invention, the field-sensordevice can comprise a plurality of calibrated field sensors. The fieldcan be a magnetic field, an electric field, a pressure field or agravitational field. The reference field sensor and the one or morecalibrated field sensors can be biased at a common reference voltage.

In an embodiment, the field-sensor device comprises a control circuitthat controls the adjustable current bias of the calibrated field sensorat a calibrated current different from the reference current so that thecalibrated field sensor provides a calibrated sensor signalsubstantially equal to the reference sensor signal. The control circuitcan comprise a reference current source that provides the referencecurrent and an adjustable current source that provides the adjustablecurrent. The control circuit can comprise a comparator that compares thereference sensor signal to the calibrated sensor signals. The controlcircuit can comprise a converter that converts the reference sensorsignal, the calibrated sensor signal or both the reference sensor signaland the calibrated sensor signal into digital signals.

In some embodiments, the reference field sensor, the calibrated fieldsensor or both the reference field sensor and the calibrated fieldsensor are digital sensors providing their sensor signals in digitalform, or the reference sensor signal, the calibrated sensor signal orboth the reference sensor signal and the calibrated sensor signal aredigital signals. In other embodiments, the reference field sensor, thecalibrated field sensor or both the reference field sensor and thecalibrated field sensor are analog sensors providing their sensorsignals in analog form or the reference sensor signal, the calibratedsensor signal or both the reference sensor signal and the calibratedsensor signal are analog signals.

A method of calibrating the field-sensor device in an embodiment of thepresent invention comprises: providing a field (for example acalibration field) to both the reference field sensor and the calibratedfield sensor; comparing the reference sensor signal and the calibratedsensor signal provided by the reference field sensor and the calibratedfield sensor, respectively; if the reference sensor signal and thecalibrated sensor signal are substantially not equal, biasing thecalibrated field sensor with an adjusted current in response to thecomparison signal; and repeatedly comparing the reference sensor signaland the calibrated sensor signal and adjusting the calibrated sensorsignal until the calibrated sensor signal substantially equals thereference sensor signal, for example within a desired tolerance.

In one embodiment the above-mentioned steps are performed beforeproviding the field sensor device to a customer and after providing thefield-sensor device to the customer the steps are performed of providingan operational field to said reference field sensor, said calibratedfield sensor or to both said reference field sensor and said calibratedfield sensor and outputting said reference sensor signal, saidcalibrated sensor signal, both said reference sensor signal and saidcalibrated sensor signal or a combination of said reference sensorsignal and said calibrated sensor signal.

A further method of the present invention, comprises operating thefield-sensor device by providing an operational field to the referencefield sensor, the calibrated field sensor or to both the reference fieldsensor and the calibrated field sensor and outputting the referencesensor signal, the calibrated sensor signal, both the reference sensorsignal and the calibrated sensor signal, or a combination of thereference sensor signal and the calibrated sensor signal.

Methods of the present invention can further include processing thereference sensor signal and the calibrated sensor signal to form agradient signal. Such a gradient signal is generated through thedifference between the reference sensor signal and the calibrated sensorsignal. If both sensors have the same sensitivity, then this differencerepresents the difference in the local field sensed by each sensor,which, divided by the known distance of both sensors, is the fieldgradient.

In some configurations, the field-sensor device comprises a plurality ofcalibrated field sensors and a method of the present invention comprisesprocessing two or more of the calibrated sensor signals from theplurality of calibrated field sensors to provide a gradient signal. Inanother configuration, the field-sensor device comprises a plurality ofcalibrated field sensors and a method of the present invention comprisescombining the calibrated sensor signal from each calibrated field sensorto provide a combined sensor signal. In some embodiments, the referenceor calibrated field sensors measure different vector components of afield having both direction and magnitude, or the reference orcalibrated field sensors measure different combinations of the fieldvector components (e.g., B_(x), B_(y), B_(z) magnetic field components).

Embodiments of the present invention provide a field sensor device withsensor field signals having improved accuracy and precision and improvedstray field rejection.

For purposes of summarizing the invention and the advantages achievedover the prior art, certain objects and advantages of the invention havebeen described herein above. Of course, it is to be understood that notnecessarily all such objects or advantages may be achieved in accordancewith any particular embodiment of the invention. Thus, for example,those skilled in the art will recognize that the invention may beembodied or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

The above and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, wherein like reference numeralsrefer to like elements in the various figures.

FIG. 1 illustrates a simplified schematic of an illustrative embodimentof the present invention.

FIG. 2 illustrates a flow chart illustrating a method according to anembodiment of the present invention.

FIG. 3 represents a schematic of an illustrative embodiment of thepresent invention.

FIG. 4 represents a graph illustrating the performance of an embodimentof the present invention.

The features and advantages of the present disclosure will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The figures are not drawn to scalesince the variation in size of various elements in the Figures is toogreat to permit depiction to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims.

Furthermore, the terms first, second and the like in the description andin the claims, are used for distinguishing between similar elements andnot necessarily for describing a sequence, either temporally, spatially,in ranking or in any other manner. It is to be understood that the termsso used are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other sequences than described or illustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepresent invention, the only relevant components of the device are A andB.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly it should be appreciated that in the description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure or descriptionthereof for the purpose of streamlining the disclosure and aiding in theunderstanding of one or more of the various inventive aspects. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the claimed invention requires more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive aspects lie in less than all features of a singleforegoing disclosed embodiment. Thus, the claims following the detaileddescription are hereby expressly incorporated into this detaileddescription, with each claim standing on its own as a separateembodiment of this invention.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

It should be noted that the use of particular terminology whendescribing certain features or aspects of the invention should not betaken to imply that the terminology is being re-defined herein to berestricted to include any specific characteristics of the features oraspects of the invention with which that terminology is associated.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

Embodiments of the present invention provide field sensor devices havingimproved stray field rejection and reduced variability. Referring to thesimplified circuit schematic of FIG. 1, a field-sensor device 99comprises a reference field sensor 10 and one or more calibrated fieldsensors 20. The reference field sensors 10 are biased with a referencecurrent provided by a reference current source 12 and the one or morecalibrated field sensors 20 are each biased with an individuallyadjustable current provided by an adjustable current source 22. Anadjustable current source can change the provided current to differentcurrent magnitudes, for example under the control of a control circuit30. The reference field sensor 10 provides a reference sensor signal 14in response to a field and the one or more calibrated field sensors 20each provide a separate calibrated sensor signal 24 in response to thefield. For clarity of exposition, the various control and sensor signalsare not individually distinguished in FIG. 1. The reference sensorsignal 14 and the calibrated sensor signals 24 can be connected to thecontrol circuit 30 or an external system (not shown for clarity) by awire 80 or a collection of wires 80 (a bus) and can be differentialsignals. Although shown as separate elements, the reference currentsource 12 and the adjustable current sources 22 can be considered to bea part of or are controlled by, the control circuit 30 and the controlcircuit 30 can comprise the reference current source 12 and thecalibrated current sources 22. Furthermore, the reference field sensor10 and one or more of the calibrated field sensors 20 or theirsupporting circuitry can be integrated into a common circuit or are partof a common device or structure and can be disposed on a common devicesubstrate and separated by a distance equal to or less than 10 mm, 5 mm,2 mm, 1 mm, 0.5 mm, 0.2 mm, 0.1 mm or 0.05 mm so that they can morereadily sense the same field.

When the adjustable current of each of the one or more calibrated fieldsensors 20 is equal to the reference current, the reference sensorsignal 14 is substantially not equal to the calibrated sensor signals 24produced by the one or more calibrated field sensors 20. Thus, the oneor more calibrated field sensors 20 produce a different sensor signalthan the reference field sensor 10 when exposed to the same field andbiased with the same current so that the calibrated field sensors 20 andthe reference field sensor 10 are mismatched at a common current bias,for example due to manufacturing and materials variation or variationsin mechanical stress. In contrast, when the calibrated field sensor 20is biased at a calibrated current different from the reference current,the calibrated field sensor 20 provides a calibrated sensor signal 24substantially equal to the reference sensor signal 14.

By substantially equal is meant within the manufacturing and materialsvariation of the field sensor device 99 or a desired tolerance for thefield sensor device 99. For example, a difference of 0.1% between thereference sensor signal 14 and the calibrated sensor signal 24 orbetween the different multiple calibrated sensor signals 24 can beacceptable for a field sensor device 99 specification and thus withinthe meaning of “substantially equal” in that case although a 1%variation is not acceptable and thus is no “substantially equal”. Thedesired tolerance for “substantially equal” can depend on the magnitudeof a common versus the differential portion of a field. The common fieldindicates the portion of the field that is the same for the variousfield sensors (reference and one or more calibrated field sensors 10,20) in the field sensor device 99. The differential field is the portionof the field that is different for the various field sensors in thefield sensor device 99. Since the various field sensors are disposed indifferent locations in the field sensor device 99, a non-uniform fieldcan produce different field measurements at the different locations ofthe various field sensors. The calibration (selection of adjustedcurrent for each calibrated field sensor 20) should be based on matchingthe bias currents required to provide an identical sensor signal fromthe various field sensors in response to the common (uniform) field. Ifthe different part is zero (the field gradient is zero), then the fieldis uniform for the various field sensors in the field sensor device 99.If the field is not uniform (has a gradient) and a field gradientmeasurement is desired, the gradient is the difference in the fieldmeasurements of the various field sensors, calculated by a subtractionof one field sensor signal from another field sensor signal and caninclude combined differences between the various field sensors.

According to embodiments of the present invention, the stray field is afield portion common to both the reference field sensor 10 and the oneor more calibrated field sensors 20. Thus, the calibrated field sensors20 are trimmed through the adjustable current provided by the adjustablecurrent sources 22 to provide a calibrated sensor signal 24 matching thereference sensor signal 14. The different bias currents provided by theadjustable current sources 22 can differ from the reference current, forexample by a few percent. Hence, according to embodiments of the presentinvention, the field-sensor device 99 incorporates an internalcalibration structure, the reference field sensor 10, which can beidentical to the calibrated field sensors 20.

The adjustable current sources 22 can comprise a variable active orpassive electronic component, for example a variable resistor,capacitor, rheostat, potentiometer, switch or switch array and, forexample, can be mechanically or electronically set or controlled. In animplementation of the field sensor device 99 of the present invention, acontrol circuit 30 controls the adjustable current bias of thecalibrated field sensor 20 at a calibrated current different from thereference current so that the calibrated field sensor 20 provides acalibrated sensor signal 24 substantially equal to the reference sensorsignal 14 when exposed to the same, common field.

Any one, all of, or any combination of the control circuit 30, thereference field sensor 10, the reference current source 12, the one ormore calibrated field sensors 20, and the one or more adjustable currentsources 22 can comprise analog or digital electrical circuits providingcorresponding analog or digital signals, with or without electromagneticor magnetic components, and the reference sensor signal 14 or thecalibrated sensor signal 24 can be analog or digital signals. Thecontrol circuit 30, the reference field sensor 10, the one or morecalibrated field sensors 20, the reference current source 12, and theone or more adjustable current sources 22 can be electricallyinterconnected through one or more wires 80. The control circuit 30 canfurther comprise circuits such as a current control circuit 32, acomparator circuit 34 such as an operational amplifier configured as acomparator, a converter circuit 36 such as an analog-to-digital ordigital-to-analog circuit, or other circuits useful in the field sensordevice 99. The converter circuit 36 can convert the reference sensorsignal 14, the calibrated sensor signal 24 or both the reference sensorsignal 14 and the calibrated sensor signal 24 to digital signals fromanalog signals or from analog to digital signals, or both. The controlcircuit 30 can include a storage circuit for storing any one or more ofthe reference sensor signal 14, the calibrated sensor signal 24 or anyconverted signals. The circuits can be, for example, silicon circuits,either analog circuits or digital circuits, for example CMOS circuits.

The control circuit 30 can be a discrete or integrated circuit or caninclude both discrete and integrated components, and can be an analog,digital or mixed-signal circuit. The wires 80 can be any patternedelectrical conductor, for example, a metal, metal alloy, a conductivemetal oxide or a conductive polymer provided using photolithographicmethods and materials to connect the various components, integratedcircuit dies or circuits integrated on the semiconductor substrate.

The field can be a magnetic field, an electric field, a pressure fieldor a gravitational field and the field sensor device 99 can be amagnetic field sensor device, an electric field sensor device, apressure field sensor device or a gravitational field sensor device.

In an embodiment of the present invention and as shown in FIG. 1, thereference field sensor 10 and the one or more calibrated field sensors20 are biased at a common reference voltage. Such a common voltage biascan improve the field sensor device 99 field sensitivity. Moreover, asused herein, a current source is also a current supply that supplieselectrical current and a voltage source is a voltage supply thatprovides a voltage differential across the voltage source.

By adjusting the adjustable current using the adjustable current source22 for each calibrated field sensor 20, the calibrated field sensors 20produce calibrated sensor signals 24 more closely matched to thereference sensor signal 14 produced by the reference field sensor 10. Bymatching the sensor signals, the sensor signals or a combination of thesensor signals provide a sensor signal with reduced variability andnoise and the field sensor device 99 will therefore have improved strayfield rejection.

For example, a magnetic position sensor can have a stray-field-rejectionrequirement of up to 25 mT. Conventional differential signal designsdesigned to cancel the stray field within a limited range (for exampleusing opposing Hall-effect sensors on a 1 mm radius) provide signals inresponse to a positive or negative 10 mT magnetic field. Thus, suchprior-art designs produce signals in response to fields that are muchsmaller than the stray field requirement. Even a 1% difference betweenthe sensor signals can result in a 0.7 degree error, which isunacceptable in many applications.

In practice, it is often the case that stray fields are generated bycurrent-carrying electrical conductors (wires) in the vicinity of thefield sensor device 99. The generated stray field decays with distancefrom the wire following a 1/R function (1/distance), and is effectivelya uniform stray field when the distance between the field sensors in thefield sensor device 99 and the current-carrying wire is greater than onehundred times the distance between the field sensors themselves. Inembodiments of the present invention, the stray field is common to boththe reference field sensor 10 and the one or more calibrated fieldsensors 20. In some embodiments of the present invention, the referenceand calibrated field sensors 10, 20 are separated by approximately onemillimeter, so current-carrying wires producing an effectively uniformstray field can be separated from the field sensor device 99 byapproximately 10 cm.

According to embodiments of the present invention, the one or morecalibrated field sensors 20 (such as Hall-effect sensors) areindividually biased in the current domain. The individual bias currentfor each calibrated field sensor 20 is derived from the same referencecurrent flowing through a reference field sensor 10 (e.g., anotherHall-effect sensor). The resulting voltage of each of the one or morecalibrated field sensors 20 and the reference field sensor 10 can bemaintained close to the supply rail voltage (V_(dd)) to improve thefield sensor device 99 sensitivity, for example through a feedback loop.

In embodiments of the present invention, each calibrated field sensor 20is individually adjustable digitally with adjustment steps of 0.5%. Thiscalibration can be performed at the factory before placing the fieldsensor device 99 into service. Referring to FIG. 2, according to amethod of the present invention, a field sensor device 99 is constructedin step 100 and exposed to a uniform calibration field in step 110. Thereference sensor signal 14 and calibrated sensor signals 24 are comparedin step 120. If the reference sensor signal 14 and the calibrated sensorsignals 24 do not match within a predetermined range or tolerance (matchstep 130), the adjustable current source 22 for each calibrated fieldsensor 20 is adjusted to provide an adjusted current to each calibratedfield sensor 20 in step 140. The reference sensor signal 14 andcalibrated sensor signals 24 are compared again in step 120 and testedin step 130. The process repeats until the reference sensor signal 14and calibrated sensor signals 24 match or are substantially equal, andthe field sensor device 99 is calibrated. By match or substantiallyequal is meant having the same magnitude or value within limitations ofthe calibration process, tolerance or manufacturing process. The fieldsensor device 99 can then be placed into service and operated, forexample by selling it to a customer in step 150 and placing the fieldsensor device 99 in an operational field. Once in service, the fieldsensor device 99 can be used to measure fields and field gradients (whenmultiple field sensors are present) in optional step 160. Multiplesensed signals can be combined (for example one or more calibratedsensor signals 24 are combined or reference sensor signal 14 is combinedwith one or more calibrated sensor signals 24 in optional step 170. Whenthe signals are processed as desired, they can be output in step 180,for example to an external system that uses the field sensor device 99signals in a control system.

Referring to FIG. 3, according to embodiments of the present invention,a field sensor device 99 comprises a reference field sensor 10 biasedwith a reference current to produce a reference sensor signal 14 andfour calibrated field sensors 20 biased with an adjustable current toproduce a calibrated sensor signal 24. The reference field sensor 10 andthe four calibrated field sensors 20 can be substantially identicalsensors. The calibrated field sensors 20 can be position sensors such asa rotary position sensor that detects the rotational angle of amechanism (indicated with a curved arrow). In an embodiment, thereference sensor signal 14 and four calibrated sensor signals 24 areelectrically connected to a control circuit 30 or external system (notshown) and can be differential signals. Each calibrated field sensor 20receives an adjustable current from an adjustable current source 22comprising a transistor controlled in common (a common gate signalconnection). Each adjustable current source 22 is individually adjustedin a calibration step 140 (FIG. 2). The reference current source 12 islikewise controlled by a transistor in common with the adjustablecurrent sources 22 but is not necessarily adjustable. A comparatorcircuit 34 compares a fixed voltage provided by predetermined aseries-connected resistors R1 and R2 forming a voltage divider with thevoltage provided to the reference field sensor 10 and drives atransistor forming a feedback loop with the reference field sensor 10and the reference current source 12 control and the adjustable currentsource 22 control. A voltage control V_(bias) maintains a suitablevoltage differential between the reference current source 12 and thefeedback loop transistor. A bias selection switch switches between anoperational mode and a calibration mode.

Referring to FIG. 4, the embodiment of the present invention illustratedin FIG. 3 has been constructed and tested. The invention is demonstratedto reduce the signal variability for a magnetic positional sensor in thepresence of stray fields by a factor of three to four. As shown in FIG.4, the error in a positional signal for a rotating positional sensor isshown at a variety of different angles separated by 45 degrees. Theuntrimmed, more highly variable signal exceeds an operational limit ofone degree of rotational error at 90 and 270 degrees and at all non-zeroangles has an error greater than the less variable signal produced bythe inventive, trimmed circuit of FIG. 3, which has an average errorsubstantially less than 0.5 at all tested angles. The test was performedat 35 degrees C. with a 3 mT/mm signal (a factor of 7 less than aconventional signal) with 50 samples measured per angle, demonstratingthe superior performance of embodiments of the present invention.

In further embodiments the field sensor device 99 can be operated atdifferent temperatures or stress conditions and the adjustable currentadjusted at the different temperatures or stress conditions. Theadjustable current source 22 can be controlled by the control circuit 30to provide a different calibrated sensor current at differenttemperatures or stress conditions so that the calibrated sensor signals24 match the reference sensor signal 14 at the different temperatures orstress conditions. In such embodiments, the magnitude of the adjustablecurrents can be temperature dependent and controlled and adjusted duringoperation by the control circuit 30.

The reference or calibrated field sensors 10, 20 can be Hall-effectfield sensors or magneto-resistive sensors and can comprise a compoundsemiconductor material. Alternatively, the reference or calibrated fieldsensors 10, 20 are electric field sensors, pressure field sensors orgravitational field sensors and can, for example, incorporatemicro-electro-mechanical systems (MEMS) devices.

In embodiments of the present invention the reference field sensor 10 orthe calibrated field sensor 20 can comprise one or more sensor elements,one or more pairs of sensor elements, for example a pair of Hall-effectsensor elements. The reference field sensor 10 or the calibrated fieldsensor 20 can comprise four sensor elements arranged in two orthogonalpairs to provide redundant measurements in two dimensions. In someembodiments, the first and second dimensions are orthogonal dimensions.

In an embodiment of the present invention the field sensor device 99comprises bridge field sensors that each include at least four sensorelements. The four sensing elements can be arranged in orthogonal pairs,where each pair has a redundant sensing element, or the four sensingelements can be separate. The different sensing elements can be providedin a common technology or common integrated circuit or in differenttechnologies or integrated circuits and can be integrated into a CMOSintegrated circuit.

In some embodiments of the present invention the field sensor device 99can have multiple calibrated field sensors 20 that measure the field ina common direction. Multiple field sensor devices 99 can be combined ina system to provide measurements in different field directions. Each ofthe multiple field sensor devices 99 can be, but are not necessarily,trimmed by adjusting the adjustable current of the adjustable currentsources 22 in response to fields having different field orientations ornon-correlated directions having no cross talk.

The reference field sensor 10 and the calibrated field sensor 20 can bemagnetic sensors such as Hall-effect sensors, magnetoresistive sensorssuch as extreme magnetoresistive sensors (XMR) extraordinarymagnetoresistive sensors (EMR), giant magnetoresistive sensors GMR,tunnelling magnetoresistive sensors (TMR), colossal magnetoresistivesensors (CMR) or anisotropic magnetoresistive sensors (AMR).

Each of the reference or calibrated field sensors 10, 20 can beprovided, for example, in an integrated circuit, discrete elements or asseparate integrated circuit components (such as bare die) mounted on asensor device substrate, such as a glass, ceramic, polymer orsemiconductor substrate.

The field sensor device 99 can be electrically connected to an externalsystem (not shown) that is electrically connected through wires 80 tothe control circuit 30 or reference or calibrated field sensors 10, 20.The control circuit 30, the reference or calibrated field sensors 10, 20can be disposed on different or common substrates, surfaces or devices.

The field sensor device 99 can comprise a device substrate and thereference field sensor 10, the calibrated field sensor 20 and thecontrol circuit 30 can be disposed on the device substrate andelectrically connected with electrical conductors such as wires 80, andcan include single wires 80 or buses comprising multiple wires 80 thatcan communicate power, ground, and control or sensor signals to or fromthe field sensor device 99, the control circuit 30, the reference fieldsensor 10 or the calibrated field sensor 20. The device substrate can beany substrate having one or more surfaces on which the reference andcalibrated field sensors 10, 20 can be disposed and electricallyconnected. The control circuit 30 can also be, but is not necessarily,disposed on a surface of the substrate.

In some embodiments the device substrate is or comprises a semiconductorsubstrate and the control circuit 30 is formed in or on thesemiconductor substrate. In another embodiment the control circuit 30 isan integrated circuit disposed on the device substrate and the devicesubstrate is a dielectric or has a dielectric layer or surface. Thus,the device substrate can comprise a substrate material that is at leastpartially different from a material of the reference and calibratedfield sensors 10, 20 and is at least partially different from a materialof the control circuit 30. In some embodiments the reference andcalibrated field sensors 10, 20 comprise compound semiconductors, thecontrol circuit 30 comprises a silicon semiconductor, and the substratematerial comprises a dielectric. In another embodiment the reference andcalibrated field sensors 10, 20 comprise compound semiconductors and thedevice substrate material comprises a silicon semiconductor and thecontrol circuit 30 is formed in or as part of the silicon semiconductor.

The device substrate can be mounted on a system substrate, for example asystem substrate of another device or system. Any one of the devicesubstrate, the control circuit 30, the reference field sensor 10 or thecalibrated field sensor 20 can be a micro-transfer printed component andcomprise a fractured, broken or separated tether. The control circuit30, the reference field sensor 10 or the calibrated field sensor 20 canbe packaged integrated circuits or bare die and can be micro-transferprinted onto the device substrate and the device substrate can bemicro-transfer printed onto the system substrate.

Any of the elements of the field sensor device 99 of the presentinvention can be provided in a common circuit or provided in a commonintegrated circuit, package or share common electrical components.Alternatively, any of the elements of the field sensor device 99 of thepresent invention can be provided in separate components, for example acombination of discrete circuit components or integrated circuits. Anyof the components can be analog components, include analog-to-digitalconvertors or can be digital components or mixed-signal circuits or acombination of circuit types and electronic devices. The control circuit30 can include a CPU with a program stored in a memory, a stored programmachine, a state machine or the like.

Any one or all of the various components can be disposed on a printedcircuit board or on a semiconductor substrate, or any one or all of thevarious components can be integrated as a circuit in or on thesemiconductor substrate, or some combination of integrated circuitsprovided on the semiconductor substrate and circuits formed in or thesemiconductor substrate.

One or more integrated circuit components or elements of the fieldsensor device 99 such as the control circuit 30, can be disposed on thereference or calibrated field sensor 10, 20 as bare die deposited bymicro-transfer printing and electrically connected. Alternatively, thereference or calibrated field sensor 10, 20 can be disposed on thecontrol circuit 30 as bare die deposited by micro-transfer printing andelectrically connected. Micro-transfer printed devices can comprise abroken or separated tether as a consequence of the micro-transferprinting process. The control circuit 30 can be provided as aphotolithographically defined circuit in a semiconductor substrate andthe reference or calibrated field sensor 10, 20 can be disposed on thesemiconductor substrate as bare die and electrically connected to thecontrol circuit 30 using photolithographic processes and materials.

Embodiments of the present invention can be constructed by providing adevice substrate and disposing the reference or calibrated field sensor10, 20 and control circuit 30 as integrated circuits on a surface of thedevice substrate. The integrated circuits can be disposed on the devicesubstrate surface by micro-transfer printing them from correspondingsource wafers onto the device substrate surface by breaking orseparating tethers physically connecting the integrated circuits to thesource wafer with a stamp, adhering the integrated circuits to thestamp, and then transporting the integrated circuits to the devicesubstrate surface. Alternatively, the device substrate surface can be orinclude a semiconductor layer and one or more or any portion of each ofthe reference or calibrated field sensor 10, 20 and control circuit 30are formed in the semiconductor layer and electrically connected withany integrated circuits disposed on the device substrate surface (forexample using micro-transfer printing) using wires 80 on the devicesubstrate surface, for example by using photolithographic or printedcircuit board methods and materials. Alternatively, the control circuits30 or reference or calibrated field sensor 10, 20 can bephotolithographically defined in a semiconductor substrate.

The device substrate can be one of many substrates with one or moresurfaces capable of supporting or receiving the reference or calibratedfield sensor 10, 20 and control circuit 30, for example a glass,plastic, ceramic or semiconductor substrate with two opposing relativelyplanar and parallel sides. The device substrate can have a variety ofthicknesses, for example from 10 microns to several millimeters. Thedevice substrate can be a portion or surface of another device and caninclude electronic circuitry.

Methods of forming micro-transfer printable structures are described,for example, in the paper AMOLED Displays using Transfer-PrintedIntegrated Circuits' (Journal of the Society for Information Display,2011, DOI #10.1889/JSID19.4.335, 1071-0922/11/1904-0335, pp. 335-341)and U.S. Pat. No. 8,889,485, referenced above. For a discussion ofmicro-transfer printing techniques, see U.S. Pat. Nos. 8,722,458,7,622,367 and 8,506,867. Micro-transfer printing using compoundmicro-assembly structures and methods can also be used with the presentinvention, for example, as described in U.S. patent application Ser. No.14/822,868, filed Aug. 10, 2015, entitled “Compound Micro AssemblyStrategies and Devices”. In an embodiment the field sensor device 99 isa compound micro-assembled device. Additional details useful inunderstanding and performing aspects of the present invention aredescribed in U.S. patent application Ser. No. 14/743,981, filed Jun. 18,2015, entitled “Micro Assembled LED Displays and Lighting Elements”.

It should be understood that the order of steps or order for performingcertain action is immaterial so long as the disclosed technology remainsoperable. Moreover, two or more steps or actions in some circumstancescan be conducted simultaneously. The invention has been described indetail with particular reference to certain embodiments thereof, but itwill be understood that variations and modifications can be effectedwithin the spirit and scope of the invention.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive. Theforegoing description details certain embodiments of the invention. Itwill be appreciated, however, that no matter how detailed the foregoingappears in text, the invention may be practiced in many ways. Theinvention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure and the appendedclaims. In the claims, the word “comprising” does not exclude otherelements or steps, and the indefinite article “a” or “an” does notexclude a plurality. A single processor or other unit may fulfil thefunctions of several items recited in the claims. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage. A computer program may be stored/distributed on a suitablemedium, such as an optical storage medium or a solid-state mediumsupplied together with or as part of other hardware, but may also bedistributed in other forms, such as via the Internet or other wired orwireless telecommunication systems. Any reference signs in the claimsshould not be construed as limiting the scope.

The invention claimed is:
 1. A field-sensor device comprising: areference field sensor biased with a reference current, the referencefield sensor providing a reference sensor signal in response to a field,a calibrated field sensor biased with an adjustable current andproviding a calibrated sensor signal in response to the field, wherein,when the calibrated field sensor is biased at a calibrated currentdifferent from the reference current, the calibrated field sensorprovides a calibrated sensor signal substantially equal to the referencesensor signal, and wherein said field sensor device is arranged to beexposed, when in a calibration mode, to a uniform calibration field and,when in operational mode, to an operational field being a fieldgradient.
 2. The field-sensor device of claim 1, comprising a pluralityof calibrated field sensors.
 3. The field-sensor device of claim 1,wherein the field is a magnetic field.
 4. The field-sensor device ofclaim 1, wherein the reference field sensor and the one or morecalibrated field sensors are biased at a common reference voltage. 5.The field-sensor device of claim 1, comprising a reference currentsource arranged to provide the reference current and an adjustablecurrent source to provide the adjustable current.
 6. The field-sensordevice of claim 1, comprising a comparator that compares the referencesensor signal to the calibrated sensor signals.
 7. The field-sensordevice of claim 1, wherein the reference field sensor, the calibratedfield sensor or both the reference field sensor and the calibrated fieldsensor are digital sensors, or wherein the reference sensor signal, thecalibrated sensor signal or both the reference sensor signal and thecalibrated sensor signal are digital signals.
 8. The field-sensor deviceof claim 1, comprising a converter to convert the reference sensorsignal, the calibrated sensor signal or both the reference sensor signaland the calibrated sensor signal into digital signals.
 9. Thefield-sensor device of claim 1, comprising a control circuit arranged tocontrol the adjustable current bias of the calibrated field sensor at acalibrated current different from the reference current so that thecalibrated field sensor provides a calibrated sensor signalsubstantially equal to the reference sensor signal.
 10. A method ofoperating the field-sensor device of claim 1, comprising: a) providingthe field-sensor device including the reference field sensor and thecalibrated field sensor; and b) providing the operational field to saidreference field sensor, said calibrated field sensor or to both saidreference field sensor and said calibrated field sensor; and c)outputting said reference sensor signal, said calibrated sensor signal,both said reference sensor signal and said calibrated sensor signal or acombination of said reference sensor signal and said calibrated sensorsignal.
 11. A method of calibrating a field-sensor device, comprisingthe steps of: a) providing a uniform calibration field to both areference field sensor and a calibrated field sensor of said fieldsensor device, b) comparing a reference sensor signal and a calibratedsensor signal provided by said reference field sensor and saidcalibrated field sensor, respectively, c) if said reference sensorsignal and said calibrated sensor signal are substantially not equal,biasing said calibrated field sensor with an adjusted current inresponse to said comparing, and d) repeating steps b) and c) until saidcalibrated sensor signal substantially equals said reference sensorsignal, such that when the calibrated field sensor is biased at acalibrated current different from the reference current, the calibratedsensor signal substantially equals to the reference sensor signal;wherein said field sensor device is exposed, when in a calibration mode,to a uniform calibration field and, when in operational mode, to anoperational field being a field gradient.
 12. The method of claim 11,wherein steps a) to d) are performed before providing the field sensordevice to a customer and wherein after providing the field-sensor deviceto the customer the steps are performed of providing the operationalfield to said reference field sensor, said calibrated field sensor, orto both said reference field sensor and said calibrated field sensor andoutputting said reference sensor signal, said calibrated sensor signal,both said reference sensor signal and said calibrated sensor signal or acombination of said reference sensor signal and said calibrated sensorsignal.
 13. The method of claim 11, comprising processing said referencesensor signal and said calibrated sensor signal to form a gradientsignal.
 14. The method of claim 11, wherein the field-sensor devicecomprises a plurality of calibrated field sensors, the method comprisingprocessing two or more of the calibrated sensor signals from theplurality of calibrated field sensors to provide a gradient signal. 15.The method of claim 11, wherein the field-sensor device comprises aplurality of calibrated field sensors and comprising combining thecalibrated sensor signal from each calibrated field sensor to provide acombined sensor signal.